In U.S. Pat. No. 4,224,391, it was described that the employment of electrolytes which contain salts of strong alkali or earth alkali hydroxides with weak acids, with a slight excess of hydroxide and a pH value of between 9 and 14, reduced the solubility of zinc anode batteries. Electrolytes prepared from a mixture of alkali or earth alkali metal hydroxide solutions in water and boric acid, phosphoric acid or arsenic acid, with excess hydroxide ranging from 0.02 to 3.0 equivalents per liter of solution, produces particularly favorable anode performance. Such electrolytes substantially eliminate the dual problems of shape-charge and dendritic deposition leading to a substantial improvement in the cycle life of rechargeable alkaline batteries containing zinc anodes. In that patent the combination of the alkali or earth alkali hydroxides and selected acids or their equivalent salts, such as borates, metaborates, various phosphate or arsenate salts, was taught to result in a net stoichiometric excess of the hydroxide in the range of 0.02 to 3.0 equivalents per liter.
Considerable improvements in the charge-discharge cycle life of alkaline rechargeable batteries, such as nickel oxide-zinc batteries, have been reported based on the previous invention. See, for example, M. Eisenberg, "A new Stabilized Nickel-Zinc Battery System for Electric Vehicle Applications", Paper #830287, Soc. of Automotive Eng., Mar. 4, 1983 and M. Eisenberg and J. R. Moden, "New Stabilized Chemistry Nickel-Zinc Cells", 31st Power Sources Symp., p. 265 (Electrochem. Soc. 1984). With the new electrolyte system based on U.S. Pat. No. 4,224,391, nickel oxide-zinc cells could be cycled at 80% depth-of-discharge (D.O.D.) up to 600 or more cycles compared with a typical 50-200 cycle life for conventional cells containing 34-38% by weight potassium hydroxide solutions.
Unfortunately, as cycling proceeds of rechargeable alkaline batteries of the type disclosed in U.S. Pat. No. 4,224,391, gradually the capacity yields, expressed in ampere-hours, diminish. Typical capacity losses within 500 cycles may amount to 30-50% of the original fresh cell capacity.
Investigations have also been made combining potassium hydroxide with potassium fluoride. These are reported in Paper No. 15 of the 10th International Power Sources Symposium in Brighton, England, 1976 by N. Cenek et al and in U.S. Pat. No. 4,247,610 (1981). However, cycle life and capacity results with these have remained limited.
In the U.S. Pat. No. 4,273,841 (Carlson) a ternary electrolyte composed of potassium hydroxide (KOH), potassium phosphate (K.sub.3 PO.sub.4), and potassium fluoride (KF) is proposed in concentration ranges of 5-10%, 10-20% and 5-15%, respectively. Assuming average densities of 1.28-1.37 g/cc, these concentrations can be translated in terms of molarities as follows:
KOH: 1.14-2.44 ML; K.sub.3 PO.sub.4 : 0.603-1.291 M/L and KF: 1.10-3.54 M/L.
These electrolytes however have been found to provide quite limited capacity of the nickel oxide electrodes, both in the beginning and later on as charge-discharge cycling proceeds.